(commonly-cited photometric system for describing stars' colors)
The UBV photometric system,
also called the Johnson system or Johnson-Morgan system,
is an early-adopted, widely-used photometric system and
is a system generally assumed if no other system is specified or implied.
It specifies U, B, and Vpassbands,
which in turn specifying B-V and U-B color indices.
The magnitude calibration is (traditionally) chosen to give Vega's
apparent magnitude zero, i.e., the Vega system of calibration,
so that the B-V and U-B color indexes are zero for stars of
Vega's spectral type, i.e., A0 V stars (given
compensation for reddening).
(However, modern equipment senses Vega's small magnitude variations,
and today's Vega system uses the mean value of a number of
stars over time, aiming for a more-consistent zero-magnitude that
falls within Vega's magnitude-range.)
The UBVRI photometric system, i.e.,
Johnson-Cousins system is an extension with red and
infrared passbands, R band and I band.